In certain known types of horizontal vacuum belt filter, for example as disclosed in my earlier U.S. Pat. No. 3,870,641 dated the 11th day of Mar. 1975, the filter belt or cloth, and with it the filter cake, is displaced intermittently in discrete increments. In order to permit the displacement, the vacuum system connected to the vacuum boxes is switched off, and the filter belt or cloth, with the cake and/or slurry thereon, is displaced in a discrete increment, after which the vacuum is switched on again, this sequence of operations being repeated continuously.
In other types of known horizontal vacuum belt filters, e.g. as disclosed in U.S. Pat. No. 4,038,193 (van Oosten), the filter belt or cloth, with the filter cake, moves continuously but the vacuum boxes travel intermittently backwards and forwards therebeneath, each period of backward travel occurring during a period whilst the vacuum is switched off.
One of the main reasons for the increased use of horizontal vacuum belt filters in recent years is that they permit vastly improved cake washing efficiency compared to rotary drum and other filters. Nowadays, however, the pharmaceutical and fine chemical industries in particular tend to demand even more stringent cake washing requirements. There requirements are not only stringent in terms of ultimate cake purity but also in terms of the amount of wash liquor used. The reasons for this are easy to understand. Either the wash liquor goes to waste and may cause an effluent problem, or, the wash liquor is an expensive medium in its own right so that restriction of its use is desirable, or, the final wash liquor is a valuable product and has to be recovered through multi-stage evaporation.
Initially, reduced volumes of wash liquor were achieved by introducing multi-stage counter-current washing, but practical difficulties soon discouraged this system, further limiting the amount of wash liquor used. To achieve efficient cake washing, it is obviously necessary for the wash liquor to cover the entire width of the cake on the belt. It has been found in practice that in order to cover a continuously moving slab of filter cake a minimum volume of approximately 5 to 6 liters (1.10 to 1.32 gallons) per minute per meter (39 inches) width and more practically something approaching twice that volume is required. In theory this figure could be reduced by use of fine sprays, but in practice the spray nozzles simply became blocked or the rate of flow was insufficient to ensure adequate pressure and distribution of the liquor issuing from the nozzle. Alternatively, an overflow-weir system might theoretically provide a small rate of flow on a continuous basis. However, the laws of hydraulics are such that the minimum flow required to draw an even film over the lip of a weir is approximately the above-mentioned amount of 5-6 liters per minute per meter width. In other words, the process requirements dictated by certain manufacturers meant that the desired volume of wash liquor was considerably less than the volume necessary to achieve a hydraulic practicality.
A similar problem arises in relation to the supply of slurry to the filter belt of a horizontal vacuum belt filter. Most efficient washing, having regard to the final purity of the filtered product, is achieved when a thin, even layer of slurry is deposited on the belt. In practice, however, there is always a minimum and frequently a maximum speed at which slurry can be pumped through a pipe. If the velocity of the slurry is too slow, settling out and blockage of the pipeline occurs and if the flow is too high (as may occur in a pipe of very small diameter) damage may be caused to the fragile particles in the slurry and/or pressure may build up to an unacceptable limit. In practice, therefore, it is extremely difficult to pump slurry at a rate which, in theory, is highly desirable to obtain the correct quantity of slurry on the belt.